A head up display (HUD) for a viewer in a vehicle or comparable environment provides a transparent display through which the viewer sees visual targets in the environment. The HUD presents visual information and/or identifying markers which the viewer can see without having to divert the eyes from the visual targets seen through the HUD. This is particularly valuable when the viewer is a driver, pilot, co-pilot, navigator, crane operator, or other person with control responsibilities.
In some HUDs the visual information and/or identifying markers are virtual images, such as those reflected off the rear surface of a transparent pane (e.g., a vehicle windshield). In other HUDs the visual information and/or identifying markers are not images but rather are real object displays, such as those produced by a thin electronic display (e.g., a transparent liquid-crystal display).
For HUDs that produce real object displays, the objects are typically in the near-focus plane, in contrast to the visual targets seen through the HUD, which are typically in the far-focus plane. This is also the case for a subset of virtual image HUDs, where the reflected image is in the near focus plane. Near-focus HUDs thus introduce a parallax disparity with respect to the visual targets, as described below.
For an observed far-focus visual target, human binocular vision diverges the eyes according to the distance of the visual target, in accordance with the parallax of the visual target. A near-focus HUD highlighting marker, however, has a substantially different parallax from that of the far-focus visual target, resulting in a double image for the marker in the perceived unified binocular image, to which the viewer may not readily adapt, thereby reducing the visual effectiveness of the near-focus HUD. Currently, the only way to avoid this problem is to utilize HUDs with a reflected image in the far-focus plane, but this may restrict the HUD to span only a relatively small portion of the visual field, and may not be practical to use in certain situations. In addition, this may rule out the use of HUDs that utilize real object displays, which are typically in the near-focus plane.